1. Field of the Invention
The present invention is related to bipolar switches, and specifically, to tri-modal bipolar switches.
2. Description of the Related Art
In an ideal switch, when the switch is closed, the same signal appearing at the input of the switch instantaneously appears at the output of the switch. Related art methods for implementing switches are shown in FIGS. 1(A), 1(B), and 1(C), explained herein below.
FIG. 1(A) shows a CMOS dual device switch 1. The CMOS dual device switch 1 is the closest switch to an ideal switch of the switches shown in FIGS. 1(A)-1(C). The CMOS dual device switch 1 acts like a passive switch, with the output drive capability thereof limited by stages which precede the switch 1.
A second method of implementing a switch is a diode bridge switch 2, shown in FIG. 1(B). The diode bridge switch 2 is opened when currents I.sub.1 and I.sub.2 are both turned off. A problem with the diode bridge switch 2, however, is that for proper distortion free operation, the sink or source current for a load should be much less than the current values of either I.sub.1 or I.sub.2. Therefore, during continuous operation of the diode bridge switch 2, current is wasted.
A third method of implementing a switch is a unity gain connected operational amplifier (op-amp) 3 with power down capability, shown in FIG. 1(C). A drawback, though, of the unity gain connected op-amp 3 is that trade-offs must be made between having low standby current, high current drive capability, and capacitive compensation requirements.
In switches of the related art, when an a.c. load is provided at the input of the switch and the switch is closed, the output voltage does not instantaneously equal the input voltage. If the output voltage is less than the input voltage, in the related art, a high amount of current is needed to be provided to the load to increase the output voltage to make the output voltage equal the input voltage.
Tri-modal buffer/switches implemented in CMOS or bi-CMOS are easy to realize because MOS devices act as very good switches. A typical MOS device provides a small resistance when the MOS device is turned on. Further, when the MOS device is turned off, a very small amount of leakage current is output by the MOS device. If MOS switches are used, the MOS switches should be preceded by an MOS or a bipolar buffer, for current drive capability. However, a bi-CMOS switch is more expensive to implement than is a bipolar switch. Further, a bipolar switch has a faster switching speed than does a CMOS switch.
What is needed is a high-speed/high-slew-rate tri-modal buffer/switch implemented using all bipolar devices.